1. Field of the Invention
The present invention is directed to apparatus and a method for automatically measuring the blood pressure of an individual and specifically to an apparatus and a method for separating a pulse signal from the pressure signal produced by an automatic blood pressure gauge.
2. Description of the Prior Art
A conventional automatic blood pressure gauge includes a resilient inflatable cuff and an electric pump. The pump is controlled by a microprocessor to inflate the cuff with a fluid, such as air, to a preset pressure. In addition, this automatic gauge includes a pressure transducer that measures the instantaneous air pressure levels in the cuff. The pressure signal produced by the transducer is used to determine both the instantaneous pressure of the cuff and the blood pressure pulse of the individual. This pressure signal is generally band-pass filtered, digitized and processed by the microprocessor to produce values representing the mean, systolic and diastolic blood pressure measurements of the individual.
In operation, the cuff is affixed to the upper arm area (or other extremity) of the patient and is then inflated to a pressure greater than the suspected systolic pressure, for example, 150 to 200 millimeters of mercury (mmHg). This pressure level collapses the main artery in the arm, effectively stopping any blood flow to the lower arm. Next, the cuff is deflated slowly and the signal provided by the pressure transducer is monitored to detect cuff pressure variations caused by the patient's blood pressure pulse, which is mechanically coupled to the cuff.
In general, the pulse component of the pressure signal has a relatively low amplitude, on the order of one percent of the total signal. A low-level detected blood pressure signal first appears when the cuff pressure is released to a level which allows some blood flow into the collapsed artery. As cuff deflation continues, the blood-pressure pulse signal rises in amplitude as more of the collapsed artery is allowed to expand in response to the pumping action of the heart. At some point, however, the pulse signal reaches a maximum amplitude level and then begins to decrease. This reduction in amplitude occurs as the artery becomes more fully open, the pumped blood flows without significantly expanding the artery, and the degree of mechanical coupling between the cuff and the arm of the patient is reduced.
In many automatic blood pressure measuring systems, the systolic and diastolic pressures are determined based on the cuff pressure at which the blood-pressure pulse signal exhibits maximum amplitude. Such a system is described in U.S. Pat. No. 4,735,213 entitled DEVICE AND METHOD FOR DETERMINING SYSTOLIC BLOOD PRESSURE, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. In this system, the diastolic blood pressure is determined as the cuff pressure, after the maximum pulse amplitude has been measured, at which the amplitude of the pulse signal is 70% of its maximum value.
Another exemplary system is described in U.S. Pat. No. 4,949,710 entitled METHOD OF ARTIFACT REJECTION FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY PREDICTION AND ADJUSTMENT OF BLOOD-PRESSURE DATA, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. In this system, the systolic and diastolic blood pressure levels are determined as the respective cuff pressures corresponding to the amplitude of the blood-pressure pulse signal being 60% of the maximum value, prior to reaching the maximum value; and 80% of the maximum value, after reaching the maximum value.
FIG. 1a is a plot of the pressure signal versus time for a conventional automatic blood pressure gauge. This exemplary signal is generated by the cuff being quickly inflated to a preset initial pressure, greater than the systolic pressure, linearly deflated to a pressure below the diastolic pressure and then quickly deflated the rest of the way. The blood-pressure pulse signal is shown as a waveform superimposed on the linear deflation portion of the pressure curve. For clarity, the relative amplitude of this pulse signal is exaggerated in FIG. 1a.
FIG. 1b is a plot of the blood-pressure pulse signal shown in FIG. 1a, separated from the linearly decreasing pressure signal. FIG. 1c is a plot of the peak amplitude of the signal shown in FIG. 1b. As illustrated by FIG. 1c, the amplitude of the pulse signal increases gradually until a time S, at which the linearly decreasing cuff pressure is the same as the systolic pressure of the patient. The amplitude of the pulse signal then increases at a greater rate from time S to time M at which the maximum amplitude is reached. The blood pressure level corresponding to this maximum pulse amplitude is commonly referred to as the mean arterial pressure (MAP). From this maximum amplitude, the pulse signal decreases rapidly to a time D, at which the cuff pressure is the diastolic pressure. The signal amplitude decreases from the point D until the cuff is entirely deflated.
In order to accurately determine the systolic and diastolic pressures of the patient, it is important that the amplitude of the blood pressure pulse signal component of the pressure signal be accurately determined.
In many prior-art automatic blood pressure gauges, including the two that are referred to above, the pressure signal is amplified and band-pass filtered to separate the blood-pressure pulse signal. This signal is then applied to an analog-to-digital converter which has a dynamic range matched to the maximum amplitude of the separated pulse signal.
However, the high-pass component of this band-pass filter may distort the peak-to-peak values of the blood pressure pulse signal by its transient response and its response to the pulse width of the blood pressure signal. The distortion from the high-pass filter can be reduced by having a relatively low high-pass cut-off frequency, but it may then exhibit an unacceptably long transient recovery time which may distort the individual blood-pressure pulses. This is especially true in the case of motion artifacts where large artifact pulsations are introduced to the system. On the other hand, if the high-pass cut off frequency is too high, significant components of the pulse signal may be lost. In addition, if the filter exhibits resonant behavior at any frequency, pulse signals having components which approach this frequency may be distorted by a damped oscillation at the resonant frequency.